Chlorine dioxide (ClO2) is a highly reactive yellowish-green gas molecule. Highly soluble in water, ClO2 is used in a variety of applications, such as for pulp-bleaching, as a bactericide, a viricide, an algaecide, a fungicide, a potent antimicrobial agent and a selective oxidizer. Chlorine dioxide is an effective antimicrobial even at very low concentrations and over wide range of pH.
A variety of methods are known for generating chlorine dioxide. It is typically produced commercially from aqueous solutions of chlorite-containing salts. See, e.g., U.S. Pat. No. 5,009,875 and Ullmann's Encyclopedia of Industrial Chemistry, vol. A 6, p. 496-500.
The reaction by which sodium chlorite and acid generate chlorine dioxide is exceedingly vigorous at high concentrations of reactants. If the reaction rate is not controlled, explosion can result, since chlorine dioxide is explosive in gaseous concentrations of about 10 volume percent in air. Due to its explosive nature and inherent instability, chlorine dioxide is generally not transported; it has historically been produced on-site at the time of use.
The focus of most recent efforts in chlorine-dioxide generation has been toward the creation of stable formulations and solutions. That is, precursor formulations that have a non-trivial shelf life and solutions of chlorine dioxide that, once formed, persist for weeks.
One such approach is to provide “stabilized” chlorine dioxide products. These products release chlorine dioxide, thereby creating a solution of “free” chlorine dioxide, when the pH is lowered to provide an acidic environment. See, for example, U.S. Pat. Nos. 3,123,521, 3,147,124, 4,396,102, 5,324,447 and 5,336,434.
The latter two patents referenced above, U.S. Pat. No. 5,324,447 and U.S. Pat. No. 5,336,434, disclose systems for cleaning contact lenses. U.S. Pat. No. 5,324,447 discloses a two-part chlorine-dioxide releasing system, wherein one part is a tablet activator and the other is a stabilized chloride dioxide product. This product is available from Bio-cide International Inc. under the trademark PUREGENE®. U.S. Pat. No. 5,336,434 also discloses a two-part chorine-dioxide releasing system wherein one part is the stabilized sodium chlorine dioxide (i.e., PUREGENE®) and the other part (the activator) is a tablet consists of a reducing agent (i.e., sodium thiosulfate). Dropping the activator tablet into the stabilized chlorine dioxide solution generates free chlorine dioxide via a reducing mechanism. No extra acid is added, thereby avoiding a pH change. The activator tablet also includes an enzyme for removing certain debris from a contact lens. Because many enzymes are inactive in the presence of chlorine dioxide, the release of the activator (reducing) component is delayed via a delayed release component, such as certain cellulose compounds.
U.S. Pat. No. 7,229,647 discloses storing pre-generated chlorine dioxide in sodium/potassium polyacrylic acid water-absorbent polymer.
Another common approach for chlorine-dioxide generation is to prepare compositions comprising dry pre-mixed solid ingredients, wherein the reaction is activated by water vapor or liquid water. As previously indicated, if the composition contains an alkali metal chlorite salt (e.g., sodium chlorite, etc.) and acid, it will react violently, even when exposed to a relatively small amount of water vapor or liquid water. As a consequence, preparations that are activated by water must control the supply of water to the solid reactants. This is often done using some type of physical barrier.
U.S. Pat. No. 5,719,100, for example, discloses the production of chlorine dioxide in an aqueous solution from a tablet comprising a composition of sodium chlorite and an acid activator, wherein the composition requires a coating that segregates the sodium chlorite and acid component. U.S. Pat. No. 6,238,643 discloses separating the reactants—a metal chlorite and an acid-forming component—from liquid water by a membrane (i.e., a Tyvek® bag/sachet). The membrane permits controlled passage of liquid water and/or water vapor. Chlorine dioxide is generated when water passes through the membrane. The chlorine dioxide that is generated passes out through the membrane into liquid water to produce the desired aqueous chlorine dioxide solution.
Another approach for controlling the reaction rate is to segregate reactants in a shell or compartmentalized structure and use a wick, etc., to control the rate at which the reactants are brought into contact with one another. For example, U.S. Pat. No. 5,091,107 discloses bringing an aqueous chlorite composition into contact, at a controlled rate via a wick, etc., with an absorbent pad containing acid or other reactant that will react with the chlorite to form chlorine dioxide. Similarly, U.S. Pat. No. 6,764,661 discloses a membrane shell that defines a compartment. The compartment includes reactants that generate chlorine dioxide gas when exposed to water. A wick extends into the compartment for absorbing water and transporting water into the compartment so that the chemical(s) in the compartment dissolve in the water and produce chlorine dioxide. U.S. Publ. Pat. Application 2009/0142235 discloses a disinfectant-generating device that includes a membrane shell defining at least two compartments. Each of the compartments includes at least one dry reactant capable of reacting and producing a disinfectant upon exposure of the device to water or ambient moisture. Each compartment is provided with an outer membrane defining walls of the device, an inner membrane providing physical separation of the dry reactants, and a wick.
U.S. Pat. Nos. 5,974,810, 6,077,495, 6,294,108, 7,220,367 disclose methods, compositions and systems for generating chlorine dioxide gas in a controlled-release manner.
A further and popular approach to generating chlorine dioxide is to include a free halogen (chlorine) source, such as sodium dichloroisocyanuric acid (NaDCC), in the composition. When exposed to water, the composition releases chlorine dioxide. See, e.g., U.S. Pat. Nos. 4,104,190, 6,432,322, 6,699,404, 7,182,883, 7,465,410, and U.S. Publ. Pat. Appls. 2006/0169949 and 2007/0172412.
An additional method for the controlled generation of chlorine dioxide is disclosed in U.S. Pat. No. 6,921,743, wherein chlorine dioxide is generated electrochemically; an acid activator is not required.
The prior-art devices and methods discussed above suffer from a variety of drawbacks. For methods that use a sachet/bag to generate chlorine dioxide, when the bag is placed in water, chlorine dioxide is generated at a rate that is often greater than the rate at which it permeates out of the sachet. As a consequence, a high concentration of ClO2 gas can result inside the sachet, resulting in explosion.
Prior art devices and methods that use membranes are susceptible to premature activation by water or water vapor. This results in a reduced shelf life unless sufficient steps, such as providing an air-tight foil seal, are taken to prevent exposure to ambient moisture or water. But even when such a seal is used, after a few months of storage, the foils tend to crack and lose their seal.
Many of the prior-art compositions include calcium or magnesium salts as a desiccant to preserve self life. In the absence of these salts, premature release of chlorine dioxide typically occurs, which can result in explosion. If the chlorine dioxide is intended for use in a soapy solution, the presence of these salts is detrimental because they add hardness to the water. Furthermore, due to the heat of hydration, the presence of calcium or magnesium salts undesirably adds more heat to the already quite exothermic chlorine-dioxide-generating reaction between sodium chlorite and acid.
Tablets generally produce chlorine dioxide at a greater rate than membrane devices because the tablet does not have a membrane to restrict chlorine dioxide from escaping into solution. But the quality of the resulting ClO2 is questionable because unconverted reagents are present along with the ClO2. As previously discussed, many compositions include the free-chlorine source sodium dichloroisocyanuric acid (NaDCC) for generating chlorine dioxide. Such compositions generate little or no chlorine dioxide in the absence of NaDCC. But for many applications, the presence of NaDCC with the generated ClO2 is undesirable.
A need therefore remains for a simple, convenient, and safe way to generate chlorine dioxide gas or solutions at high yield and with high quality, and from a composition that has a long shelf life.